I believe that everything happens for a reason. People change so that you can learn to let go, things go wrong so that you appreciate them when they're right, you believe lies so you eventually learn to trust no one but yourself, and sometimes good things fall apart so better things can fall together.
The trouble is that people seem to expect happiness in life. I can't imagine why; but they do. They are unhappy before they marry, and they imagine to themselves that the reason of their unhappiness will be removed when they are married. When it isn't they blame the other person, which is clearly absurd. I believe that is what generally starts the trouble.